The invention relates to a method of producing molten glass and to a glass melting tank for carrying out the method.
In the manufacture of glass in tank furances, unmelted batch material is fed onto an established bath of molten glass at one end of the tank where it is melted. The molten glass which forms from the batch material passes from the inlet end of the tank sequentially through a melting zone, a refining zone and a conditioning zone before being drawn off from an outlet end of the tank for use in a glass forming process in known manner.
Conventionally heat is supplied for melting and refining the glass by the combustion of liquid or gaseous fuel above the glass surface, or by electric heating within the body of the glass or by a combination of both methods.
A rising temperature gradient is normally arranged in the molten glass along the melting zone of the tank, by control of the energy input along the tank length, the temperature reaching a maximum at a so-called hot spot. Downstream of this position the energy input is controlled to cause a falling temperature gradient in the molten glass. The effect of these gradients is to give rise to convection currents which return hot glass in the upper layers underneath a blanket of unmelted batch material from the hot spot towards the filling end. At the same time glass in the upper layers downstream of the hot spot is carried forwards towards the conditioning zone and colder glass in the lower layers flows back towards the hot spot. These convective flows serve to homogenise the glass and the colder lower layers of glass in the refining zone prevent furnace bottom refractories reaching a temperature sufficiently high for rapid chemical attack and erosion.
It is difficult in practice to obtain completely homogeneous glass in a glass melting tank particularly when the output from the tank is high in relation to its size since high energy inputs are necessary to melt and refine the glass whilst high temperature gradients are required to maintain the convective circulation at high enough levels. As the output of the tank is increased, more heat passes through the tank with the glass to the conditioning zone where it has to be removed to bring the glass to a satisfactory thermal condition for processing.
The glass in the conditioning zone is generally cooled by air blown across the free surface of the glass but if this surface cooling is sufficiently large, hotter glass from within the body of the glass rises disrupting the smooth flow of layers of glass, which varies slightly in composition, and results in these layers deviating from a state generally parallel to the major surface of the glass in the final product. This gives rise to optical faults and the disruption of flow is termed "inversion".
It is known that in a conventional tank furnace with one exit on the central line of the tank only a relatively small proportion of the glass being melted, refined and conditioned travels directly to the outlet end of the forming process. This comes from within a narrow region in the conditioning zone about the central line of the furnace and the remainder moves under convective flow towards the walls of the tank where it sinks and returns towards the inlet end forming a return flow as discussed above.
It is an object of the present invention to provide an improved method and apparatus for manufacturing molten glass in which the forward flowing glass which is advancing towards the conditioning zone is stirred and selectively cooled so as to achieve improved results.